ATF Form 4473 is required to be filled by the buyer at the time of purchase.

Brian Guerra, owner of Lone Star Guns and Armas de Guerra, explained, "It's a legal federal document that is used to verify their document that is used to verify their identity and to make sure that they can legally obtain a firearm."

Once the form is filled out, it's turned over to the gun store employee.

At the Gun Rack in McAllen, Jose Cruz, then-manager, told agents he falsified at least one form by adding the purchase of a second weapon when the buyer only bought one. Then, he sold the weapon to someone else without documenting the purchase.

"That additional information is in violation of how the form is supposed to be filled out. So, he was actually committing perjury in a sense," says Guerra.

Gun store employees are in a position to tamper with the forms when they're returned to them by the buyer. It can be done by either adding a weapon on the form already filled out by the customer, or forging a new one with the customer's information. However it's accomplished, the consequences boomerang to the form.

"If a gun is used in a crime, the serial number is then traced to the manufacturer, which is then that information of the manufacturer will tell ATF, the inspecting agency, where that gun was sold to, usually a wholesaler. Then that wholesaler will then tell which dealer they sent it to. Then the dealer will then say which person purchased that firearm," explained Guerra.

If the gun buyer didn't buy it, that leaves the store looking suspicious to ATF agents.

Straw purchases, where gun buyers lie on the same form to buy guns for other people, happens all the time. Guerra says when a store engages in this crime, it hurts the industry as a whole.

He says, "This person was altering documents to distribute weapons to people who would not normally be able to get them. And not his own documents, other people's documents. So, it's a massive violation of public trust.”

Cruz is maintaining his innocence. He is also facing a separate charge for attempting or exporting up to six rifles from the United States. Jury selection is scheduled to start Feb. 4.